# A question regarding SoapCalc and measuring out your oils

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#### theplasticfantasty

##### Well-Known Member
So I want to see if I'm understanding something correctly, I hope you all can help me out here

When you're trying to calculate how much oil you'll need for a mold, you take your mold measurements and multiply length X width X height, and then multiply that number by .40. I understand that the .40 represents the 40% that your water/lye solution takes up in your recipe. I also understand that, depending on how much you water discount, that percentage can change

On SoapCalc's recipe calculator, one of the variables you can change is the "water as % of oils". By default it's set at 38. My question is this: is this percentage the same percentage you're multiplying your mold measurements by to figure out your needed oil weight? So if it's set at 38% in the SoapCalc, you'd instead multiply your measurements by .38 instead of .40?

I appreciate everyone's help and patience in explaining what's probably really obvious! I just want to be sure I'm teaching myself and understanding recipe formulation properly

#### MoonGal

##### Member
Hello

I have a mold with inner dimensions of 10.25 x 2.75 x 3. I use a recipe with 36 oz. by weight of oils and that fills the mold plus a little extra (I turn that extra into soap dough). Using the formula, I would have used 33.8 oz. of oils. So it looks like the formula you are using is pretty close.

The 40% is a basic starting measurement that includes about 35% for water weigh, 5% for lye weight. But each oil needs a different amount of lye to saponify, so your lye amount and weight will vary.

Since I don't mind having extra soap for soap dough, I just make a good guess and then keep my soap recipes for future comparison.

Oh, as a side note, I do NOT use the water as a percent of oil weight metric. Not ever. I use a ratio when determining my lye concentration, and usually stick with a 2:1 water lye ratio. My soap doesn't partially gel, it cures in about 4-6 weeks, and I'm pretty happy with it.

In conclusion, I do not think the 38 that you are referring to is being used to determine the volume of your mold, I believe it is just for calculating lye. I hope that if this is not correct that someone else can fill-in the gaps

#### theplasticfantasty

##### Well-Known Member
In conclusion, I do not think the 38 that you are referring to is being used to determine the volume of your mold, I believe it is just for calculating lye. I hope that if this is not correct that someone else can fill-in the gaps

Thanks for your response! I really thought I'd figured something out here but they appear to be unrelated hah. Your response was very informative though, I appreciate you going into detail

Thank you for the reference link! That was the page I found when looking for how to calculate my oil weight. I guess my question was whether or not that calculation had anything to do with your water percentage, but I guess not.. I'm still not totally sure, to be honest

From what I've read, that 38% listed on SoapCalc is to account for your water/lye weight; basically the other half of what'll be going in your batter besides the oils (not including any other additives). Is that not the same percentage (or decimal) you're using in your mold size calculation?

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#### DeeAnna

##### Well-Known Member
The calculation in this article gives you an estimate of how big of a soap batch is needed to fit a particular mold, assuming you've never used the mold before and need some guidance. Rather than give a total batch weight, the answer is given as a combined weight of fats, which is easier for people to work with.

Once you know the estimated total weight of fats, you then go back to a soap recipe calculator to figure out an actual recipe.

This estimated weight is based on some assumptions about how the soap batch is designed, including the amount of water used. Read the "extra credit" section at the end to understand these assumptions better. The answer you get from this estimate isn't going to be perfect, but it gives a person a reasonable number to start with.

Once you've made soap in a particular mold using the estimated total fat weight, you will know whether you made too much batter or too little. You can tweak the total weight of fats higher or lower so the batter fills the mold the way you want.

***

38% water as % of oils is one way to calculate the amount of water for a given batch. It has NOTHING to do with the weight of NaOH required for the batch.

The type of fats and their weights as well as the superfat setting are the ONLY things that control the weight of NaOH.

Water as % of oils, lye concentration, and water:lye ratio are the three settings a person can use to control the weight of water, and water only. Pick one, get used to it, and stick with it.

MoonGal gave good advice -- either lye concentration or water:lye ratio are the best choices. I never use "water as % of oils" in my soap making. I have more info here: Table of contents | Soapy Stuff

#### theplasticfantasty

##### Well-Known Member
I'm always so appreciative of your responses @DeeAnna, they're always so thorough and well explained, thank you!

After searching for a little bit I came across this reply by a user on here from some time ago:

I recently found out why you multiply the volume of the mold by .4. The .4 is actually the percent of Water as a % of oil. It looks like the formula here uses 40% instead of the standard 38%.

So, if you are using 38% you would multiply the volume of the mold (width X height X length) in cubic inches by .38. This will give you the amount of oil the mold will hold when 38% water is used.

So, if you are using a different amount of water, the mold will hold a different amount of oils. However, the mold will always same the same volume of soap.

Which appears to answer the exact question I had. Are they correct in what they're saying?

#### AliOop

I agree with what Tim884 wrote except the line that says "if you are using a different amount of water, the mold will hold a different amount of oils." I don't know of anyone who ever changes the amount of oils in a recipe based on how much water they are using. That's completely illogical. That is a very confusing way of saying (I THINK) that using a different amount of water will change your total batch size, and thus affect what fits into the mold.

This highlights part of why I disagree with using the "water as % of oils" option for any soap recipe. Although it may simplify the process of estimating how much will fit into your mold, you can end up with some very funky results when scaling up or down. And it puts the focus on the relationship between water and oil, which is almost irrelevant. The important relationships are oils to NaOH, and water to NaOh. Only when those numbers are correct, do I think about how much water to add or reduce, and then only for purposes of time to trace, reducing ash, gelling, glycerine rivers, etc.

As DeeAnna noted, it is much better to use lye concentration, or water:lye ratio. Unlike "water as % of oils," either of those choices will give you consistent results, regardless of batch size.

#### DeeAnna

##### Well-Known Member
"...multiply the volume of the mold by .4. The .4 is actually the percent of Water as a % of oil. It looks like the formula here uses 40% instead of the standard 38%. ..."

Which appears to answer the exact question I had. Are they correct in what they're saying?

No, Tim884 was not correct. The math used to derive the 0.40 Rule has nothing to do with "38% water as % of oils".

I gather you have not yet read the "extra credit" section in my article. I derive the 0.40 rule for inches and ounces and the 0.70 rule for grams and centimeters using rational soap making decisions and practical knowledge of science and mathematics. It's not just me making wild-a** guesses pulled out of thin air.

#### AliOop

Thanks, I should have read further up the thread to get the link to your article. I can't say I understand all the math, but I do TRUST your math.

Unlike my typical preference for letting someone else do the math, I do figure my mold volumes by hand, using the .70 rule since I use grams. While I have used the recipe re-sizer on SoapmakingFriend, unfortunately, it made 2.5 times the amount of soap needed. Since it was a good-sized mold to begin with, I made a LOT of soap that day. While it could be (and most likely was) some kind of user error, others on SMF have posted about similar wacky results using that particular re-sizing tool. And I double- and triple-checked that I had entered the mold dimensions correctly, and I had.

#### DeeAnna

##### Well-Known Member
Thanks, I should have read further up the thread to get the link to your article. I can't say I understand all the math, but I do TRUST your math.

I was responding to the OP, since they want to know how the 0.40 Rule was derived. I have to say poor Tim448 seem to have a rather shaky understanding of what he was talking about.

I've read some nutty theories about this rule. They tend to be based on the "it looks reasonable to me, so therefore it must be true" method of proof. I decided to make a rigorous proof based on math and science facts, not just wishful thinking.

I don't think anyone needs to slog through my proof if they're not interested -- it's only there for those who want to know more about the mathy details.

"...While I have used the recipe re-sizer on SoapmakingFriend, unfortunately, it made 2.5 times the amount of soap needed. ...others on SMF have posted about similar wacky results using that particular re-sizing tool...."

Yikes! Thanks for sharing the cautionary tale, @AliOop. I know these resisizers are available, but I've never evaluated them personally so I don't know how well they work or what a person has to do to use them. Obviously there's room for improvement.

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#### AliOop

I actually enjoy the math concepts; it's the execution details where I tend to mess up (lots of transposition errors). But if I at least understand the why and how, that helps me know if I'm operating in the reasonable zone, or headed for a big fail.

I'm still scratching my head about the results on the re-sizing tool. The first time I used it, the results were spot on. The second time was for cylinder molds, and I had over 2x the amount of batter needed. The third and final time was for a brand new slab mold, and it gave me 2.5x the amount of batter I needed. Either one of their updates adversely affected that feature, or there is another setting that I need to adjust. If the latter is the case, for the life of me, I can't figure out what that might be.

#### lshone

This question is pretty common. Here’s a tool that I built over the last few days to calculate the amount of oil needed to create a batter to fill a soap mold (part of my New Year's resolution to learn java script).

==> Simple Soap Volume Calculator

You will still need to work out the volume for your mold, and use a soap calculator, like soapmakingfriend.com, to calculate the SAP value. It only works with NaOH recipes for now.

To get the saponification value for your recipe, you’ll need to run the recipe through a soap calculator, like soapmakingfriend.com. Set the unit of measure to metric and the superfat value to 0 and look at the results. You're interested in two numbers, the NaOH weight and the Oil weight. Divide the NaOH weight by the Oil weight. The result is the saponification value for your recipe. This will be unique for every recipe and is the reason you need to use a soap calculator.

Fair warning: Use at your own risk! For me this is a learning tool, so it will probably change over time or morph into something else. Also I’m using a free google plan to host this, but if it starts costing money I’ll probably disable it.

#### lshone

I'm still scratching my head about the results on the re-sizing tool. The first time I used it, the results were spot on. The second time was for cylinder molds, and I had over 2x the amount of batter needed. The third and final time was for a brand new slab mold, and it gave me 2.5x the amount of batter I needed. Either one of their updates adversely affected that feature, or there is another setting that I need to adjust. If the latter is the case, for the life of me, I can't figure out what that might be.

If you are using the cylinder option be aware that it mistakenly divides the radius in half and gives half the volume you should get. If you enter the diameter of the cylinder you should get the correct volume.

Soapmaking friend uses the ".40" method to calculate the oil volume, so your results will vary depending on the recipe.

#### AliOop

@lshone that is a cool tool! thank you for sharing it.

#### Monab

##### Active Member
So I want to see if I'm understanding something correctly, I hope you all can help me out here

When you're trying to calculate how much oil you'll need for a mold, you take your mold measurements and multiply length X width X height, and then multiply that number by .40. I understand that the .40 represents the 40% that your water/lye solution takes up in your recipe. I also understand that, depending on how much you water discount, that percentage can change

On SoapCalc's recipe calculator, one of the variables you can change is the "water as % of oils". By default it's set at 38. My question is this: is this percentage the same percentage you're multiplying your mold measurements by to figure out your needed oil weight? So if it's set at 38% in the SoapCalc, you'd instead multiply your measurements by .38 instead of .40?

I appreciate everyone's help and patience in explaining what's probably really obvious! I just want to be sure I'm teaching myself and understanding recipe formulation properly
I just put 35 for my oil weight in my soap calculator, Superfat at 5, lye concentration at 33 and fragrances at zero. Which, should put me at 49 oz. That is the amount I make for the Amazon mold the pink and purple one. You're oil weight is just the amount of oil you are using. The supperfat is the amount of oils you want left over in your soap. My soap calculator oil weight default is set at 16 but my water(lye concentration) is set at 33% which is normal rate soapera use. I use those app to calculate my percentage for lye, water. There are app available that can do the math for you. I posted some book i read on repeat. It free if you have kindle and soap queen tutorials. I'm guessing you not understanding the how to calculate water lye. I'm not good with math and horrible at writing. So, I'm giving you what I used to understand. It was a lot of reading. I hope is help. Water Discounting Cold Process Soap: How & Why - Soap Queen
Below is the article that actually made me understand lye concentration.

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#### theplasticfantasty

##### Well-Known Member
No, Tim884 was not correct. The math used to derive the 0.40 Rule has nothing to do with "38% water as % of oils".

I gather you have not yet read the "extra credit" section in my article. I derive the 0.40 rule for inches and ounces and the 0.70 rule for grams and centimeters using rational soap making decisions and practical knowledge of science and mathematics. It's not just me making wild-a** guesses pulled out of thin air.

I didn't mean to imply you were just guessing at numbers, I hope I didn't offend! Math has never been my strong suit so this has definitely been the most difficult aspect for me to grasp; I'm doing my best to make sure I understand. I'm grateful for you taking the time to explain, sorry I'm a little slow on the uptake! I'm always amazed at people who can work out problems like this

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#### DeeAnna

##### Well-Known Member
Oh, gosh, I'm not offended, @theplasticfantasty -- no need to worry. I was puzzled at the time about why you were wanting to discuss answers like Tim's when I'd already given you (what I thought) was a decent answer. But it was a small matter at the time and it's long since gone.

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#### theplasticfantasty

##### Well-Known Member
I totally understand that! I hope it didn't seem like I was dismissing your answers because that wasn't the case; I brought up Tim's post because I thought it phrased what I was asking better than I could just in case I was not being clear, but I realize now that wasn't really necessary lol

Your answers were brilliant. It took a minute for me to process, clearly, but I'm starting to understand that the two things I was correlating actually are not really related. The chemistry aspects of soaping fascinate me so much but they go hand in hand with the mathematical aspects and I'm not so stoked about that hah

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